As is known in the art, certain metal hydride alloy materials are capable of absorbing and desorbing hydrogen. These materials can be used as hydrogen storage media and/or as electrode materials for fuel cells and metal hydride batteries including metal hydride/air battery systems.
When an electrical potential is applied between the cathode and a metal hydride anode in a metal hydride cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion; upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron. The reactions that take place at the positive electrode of a nickel metal hydride cell are also reversible. Most metal hydride cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode.

In a metal hydride cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, felted, nylon, or polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 20 to 45 weight percent potassium hydroxide.
One particular group of metal hydride materials having utility in metal hydride battery systems is known as the ABx class of material with reference to the crystalline sites that its member component elements occupy. ABx type materials are disclosed, for example, in U.S. Pat. No. 5,536,591 and U.S. Pat. No. 6,210,498, the disclosures of which are incorporated herein by reference. Such materials may include, but are not limited to, modified LaNi5 type as well as the TiVZrNi type active materials. These materials reversibly form hydrides in order to store hydrogen.
AB5 hydride alloy materials, and in particular misch metal based AB5 alloys, are one type of ABx material, and have been used extensively as electrode materials in nickel metal hydride (NiMH) batteries. The most commonly employed AB5 alloy formulations are stoichiometric compositions including La, Ce, Pr, and Nd in the A-site and Ni, Co, Al, and Mn in the B-site. The preparation of such AB5 alloy materials generally involves an annealing step which homogenizes the composition by eliminating any secondary phases which may have been formed in the preparation of these materials. The prior art teaches that the annealing process operates to flatten and lower the plateau region in the pressure-concentration-temperature (PCT) isotherm of the materials and thereby increases the material's reversible hydrogen storage capacity, electrochemical discharge capacity, and grain size. It has further been found that the increase in grain size in general improves cycle stability but decreases high-rate dischargeability.
The present invention breaks with the prior art and recognizes that presence of a secondary phase in an AB5 material acts to improve the electrochemical properties of the bulk alloy, for example by improving capacity and/or high-rate discharge. Furthermore, it has been found that the alloys of the present invention may be prepared from metal mixtures which do not include high-priced components such as Pr or Nd. These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.